The present invention relates to a dispersion-shifted single-mode optical fiber.
Single-mode optical fibers that are referred to as xe2x80x9cdispersion-shifted fibersxe2x80x9d (DSFs) are such that, at the transmission wavelength at which they are used (which wavelength is, in general, different from the wavelength of 1.3 xcexcm for which the dispersion of silica is substantially zero), the chromatic dispersion of the transmitted wave is substantially zero; i.e. the non-zero chromatic dispersion of silica is compensated (hence, the use of the term xe2x80x9cshiftedxe2x80x9d) by increasing the index difference xcex94n between the core of the fiber and the optical cladding. This index difference makes it possible to shift the wavelength for which the chromatic dispersion is zero. It is obtained by inserting dopants into the fiber while it is being manufactured, e.g. by a modified chemical vapor deposition (MCVD) process that is known per se, and that is not described in any more detail herein. A typical value for the index difference between the cladding and the core of the fiber is 24xc3x9710xe2x88x923. The increase in refractive index in silica can be obtained by using germanium as the dopant. The terms xe2x80x9ccladdingxe2x80x9d and xe2x80x9ccorexe2x80x9d are known to the person skilled in the art; conventionally, the xe2x80x9ccladdingxe2x80x9d is the portion that extends to a diameter of 125 xcexcm. The core corresponds to the portion in which about 70% of the light energy propagates.
Such single-mode fibers must also preferably have characteristics that correspond to the requirements both of cable-makers and of system designers: firstly they must have small mode diameters, and good xe2x80x9ccablabilityxe2x80x9d (i.e. suitability for being included in a cable), involving bending capacity of the fiber and low attenuation; and secondly they must have mode diameters that are as large as possible, large effective areas, and suitable values for the zero-dispersion wavelength xcex0. When such fibers are used for wavelength-division multiplexed (WDM) transmission systems, the constraints are even more stringent.
It would be preferable to use a fiber having a xcex0 value that is greater than or equal to 1,565 nm, and advantageously that is greater than or equal to 1,585 nm, in order to avoid using a dispersion-compensating fiber (DCF), which suffers from the drawbacks of facilitating non-linear effects, of having high attenuation, and of being difficult to lay in a cable. Unfortunately, existing fibers having such xcex0 values have small effective areas or high attenuation around 1,550 nm.
Numerous index profiles have been proposed for such dispersion-shifted single-mode optical fibers. The index profile is generally described as a function of the appearance of the curve representing the refractive index as a function of the radius of the fiber. The index profile is thus said to be xe2x80x9csteppedxe2x80x9d, xe2x80x9ctrapezium-shapedxe2x80x9d or xe2x80x9ctriangularxe2x80x9d for curves representing the variation of refractive index as a function of radius that are respectively stepped, trapezium-shaped, or triangular. Such curves generally represent the ideal or reference profile of the fiber, it being possible for the constraints involved in manufacturing the fiber to give rise to a profile that is significantly different.
Early dispersion-shifted optical fibers were of the index-step, triangle, Gaussian, or xcex1 power type. The characteristics of those fibers are no longer considered to be sufficient, in particular because of their small mode diameters and of their sensitivity to bending, even though those fibers have attenuation values lower than 0.20 dB/km at 1,550 nm. A new family of fibers has thus been developed: fibers whose index profiles are trapezium-and-ring-shaped.
French Patent FR-A-2 724 234 describes a dispersion-shifted single-mode optical fiber having an index profile that is trapezium-shaped, or, in the limit, that is triangular or rectangular, with a ring surrounding the trapezium. FIG. 1 shows the appearance of the reference profile proposed in that document. As is conventional, the x-axis in FIG. 1 gives the distance r to the center of the fiber, and the y-axis gives the refractive index, defined by its percentage difference relative to the index of the cladding of the fiber. References hereinafter to the refractive index are made as relative values to the index of the cladding, as is conventional.
That fiber has an effective area of greater than 70 xcexcm2, and attenuation of about 0.20 dB/km at 1,558 nm, chromatic dispersion of about 0.7 ps/nm.km, and a xcex0 wavelength of about 1,550 nm.
European Patent Applications EP-A-0 131 634 and EP-A-0 368 014 describe a fiber of the same type, with a rectangular core surrounded by a ring.
European Patent Application EP-A-0 724 171 describes a fiber having an index profile with an xcex1 profile, surrounded by a triangular ring, as shown in FIG. 2. That fiber has an effective area of 72 xcexcm2.
In addition, European Patent Application EP-A-0 127 408 describes a fiber having an index profile with a rectangular core surrounded by two rings, the index of the rectangle and the index of each of the rings either being the same, or else said indices being different. Such a fiber has a core index that is less than or equal to the index of each of the rings. It is not apparent that such fibers make it possible to form dispersion-shifted fibers of the kind desired. In addition, the article by A. Safaai-Jazi et al., xe2x80x9cNew designs for dispersion-shifted and dispersion-flattened fibersxe2x80x9d, Proceedings of the SPIE, Vol. 1176.5, September 1989, pages 196-201, presents a fiber having an index profile having a rectangular core with two rings, the two rings having the same index, and the index of the core being greater than the index of the rings. Such a fiber has the characteristics of a fiber having flat dispersion. It is not apparent that such a fiber makes it possible to form dispersion-shifted fibers of the kind desired.
Those various known solutions do not make it possible to obtain a xcex0 wavelength value that is high or that can be chosen freely, while also conserving the characteristics suitable for good xe2x80x9ccablabilityxe2x80x9d.
The invention proposes a solution that makes it possible to maintain an effective area Aef that is large, typically above 70 xcexcm2, thereby limiting non-linear effects. The fiber of the invention also makes it possible to obtain attenuation that is low, preferably less than or equal to 0.2 dB/km, and chromatic dispersion that is generally greater than or equal to 1.5 ps/nm.km in absolute terms, e.g. about 2 ps/nm.km in absolute terms, for wavelengths in the range 1,530 nm to 1,585 nm. In addition, it makes it possible, for all these parameters, to obtain a zero chromatic dispersion wavelength xcex0 that is advantageously greater than or equal to 1,585 nm; which makes it possible (if the fiber is used for WDM transmissions) to avoid or to limit four-wave mixing and the use of dispersion-compensating fibers.
In other words, the invention proposes a method of increasing the value xcex0 of the wavelength for which chromatic dispersion is zero in a dispersion-shifted single-mode optical fiber having cladding with a given index (ns), a fiber core with an index profile that is trapezium-shaped, or in the limit that is triangular or rectangular, with a ring, and a mode diameter that is advantageously greater than or equal to 8 xcexcm, including the addition in the index profile of the fiber core of an outer annular portion of index greater than the index of the cladding and greater than or equal to, and preferably greater than, the index of the ring of the core.
The invention thus provides a dispersion-shifted single-mode fiber having:
cladding of given refractive index;
a fiber core having an index profile that is in the shape of a trapezium, a triangle, or a rectangle, together with a first ring, the index profile of the fiber core having an outer, second ring of index greater than the index of said cladding;
said fiber being characterized in that the index of said second ring is greater than or equal to the index of said first ring.
Preferably, the index of said second ring generally lies in the range 1xc3x9710xe2x88x923 to 6xc3x9710xe2x88x923.
Preferably, the index of said second ring is greater than the index of said first ring.
Preferably, the second ring has a thickness lying in the range 0.3 times the radius of the trapezium, triangle, or rectangle portion of the core to 0.8 times said radius.
It is also possible to make provision for the index of said first ring to be greater than the index of the cladding.
In an embodiment, the maximum index of said trapezium, triangle, or rectangle is generally greater than or equal to 9xc3x9710xe2x88x923.
Preferably, the maximum index of said trapezium, triangle, or rectangle has a value at least 50% greater than the value of the index of said first ring.
In an embodiment, the index of said first ring generally lies in the range 1xc3x9710xe2x88x923 to 3xc3x9710xe2x88x923.
In an embodiment, the index of the fiber between said trapezium, triangle, or rectangle and said first ring is less than or equal to the index of the cladding. Such an index generally lies in the range xe2x88x920.1xc3x9710xe2x88x923 to xe2x88x920.5xc3x9710xe2x88x923.
In another embodiment, the index between said first ring and said outer, second ring is less than or equal to the index of the cladding. Such an index generally lies in the range xe2x88x920.1xc3x9710xe2x88x923 to xe2x88x920.5xc3x9710xe2x88x923.
Preferably, the portion between said first ring and said second ring has a thickness lying in the range 0.3 times the radius of the trapezium, triangle, or rectangle portion of the core to 0.8 times said radius.
Advantageously, the index between said first ring and said outer second ring is less than or equal to the index between said trapezium, triangle, or rectangle and said first ring.
It is also possible, between said outer second ring and the cladding, to provide an annular portion of index less than the index of said second ring and the index of the cladding. Such an index generally lies in the range xe2x88x920.1xc3x9710xe2x88x923 to xe2x88x920.5xc3x9710xe2x88x923.
The fiber of the invention is such that the value of the wavelength for which the chromatic dispersion is zero is advantageously greater than or equal to 1,565 nm, and preferably greater than or equal to 1,585 nm.
Advantageously, said fiber has an effective area greater than or equal to 70 xcexcm2.
Preferably, it has attenuation less than 0.2 dB/km for a wavelength value of 1,550 nm.
In an embodiment, the fiber has a cutoff wavelength of less than 1,530 nm. It may also have chromatic dispersion of about xe2x88x922 ps/nm.km in the range 1,530 nm to 1,580 nm.